The present invention relates to copper foil for an inner layer circuit of a multi-layered printed circuit board, which has high bonding strength with prepreg, is not susceptible to haloing and facilitates an optical inspection, a method of producing the same and multi-layered printed circuit board having the same.
Conventionally, a multi-layered printed circuit board is manufactured in the following steps. First, phenolic resin, epoxy resin, polyimide resin, epoxy modified polyimide resin or thermosetting resin, such as cyanate esters, is impregnated in glass fiber fabric and nonwoven fabric, linear polyester fiber fabric and nonwoven fabric, carbon fiber and nonwoven fabric, polyamideimide fiber fabric and nonwoven fabric, or insulating base material such as paper, and the resultant structure is dried, providing prepreg. On one or bonded plural prepreg boards obtained in the above manner, a copper foil is laminated, yielding a copper-clad, laminated board. Then, the unnecessary copper foil portion on the surface of the board is etched away, providing an inner layer circuit. The copper surface of the inner layer circuit is subjected to a roughening treatment or oxidation treatment to improve the bonding strength with the prepregs. Then, an outer layer laminate is laminated on the resultant structure, and through holes are formed in the outer layer laminate for connection between the inner layer circuit and an outer layer circuit. A copper layer is formed on the through hole portions to provide conductivity, using electroless plating or the like, and the outer layer circuit is etched, yielding a multi-layered printed circuit board.
In this fabrication of a multi-layered printed circuit board, the copper foil for the inner layer circuit undergoes various processing steps and is put under strict use conditions after the formation of the circuit board. Due to recent severer processing conditions and use conditions, the copper surface of the inner layer circuit is subjected to one of the following three conventional processes in order to further improve the bonding strength between the inner layer circuit and the prepregs.
(1) After the inner layer circuit is formed using an ordinary one-side roughened copper foil, a brown or black copper oxide, called "brown oxide" or "black oxide," is formed on the copper surface using a chemical treatment liquid containing an oxidizing agent.
(2) The black oxide mentioned above is treated with a reducing agent, such as formalin, thereby forming a reduced copper (Japanese Provisional Patent Publication No. 310595/89).
(3) A copper foil with both sides roughened available on the market is to be used.
However, the copper foil for an inner layer circuit comprising the black oxide as in the case (1) has poor chemical resistance, and is susceptible to haloing. Further, the copper foil for an inner layer circuit having the brown oxide has insufficient chemical resistance and has lower bonding strength to the prepregs.
With regard to the copper foil for an inner layer circuit comprising the reduced copper as in the case (2), at the time when the inner layer circuit after its formation is subjected to automatic optical inspection using VISION 206 AOI, a product of Optrotech Co., Ltd., a high speed automated optical inspection device for printed circuit boards, which serves to provide for the inspection of the outward appearance of printed circuit boards, an erroneous inspection is often done in the AOI (automated optical inspection) due to a little color difference between the copper color of the circuit portion and the color of the insulating layer portion. This requires a significant time for confirmation of the inspection and may easily result in overlooking of circuit defects. The efficiency and reliability of the checking are therefore insufficient.
As regards the copper foil which has both sides roughened as in the case (3) for an inner layer circuit, because of a small color difference between the copper color of the circuit portion and the color of the insulating layer portion in the same way as the case of (2), the efficiency and reliability of the checking are insufficient. In addition, when the circuit board is left in the air after the formation of the circuit portions, the copper portion is likely to rust.
The above conventional processes should employ batch processing methods in which a single board is dipped in a treating tank as long as a board after being pressed or etched is treated, thus requiring a great amount of investment and processing time. The inner layer circuit to be treated still has some uncompleted circuit portions, such as through holes, or electrically unconnected portions, therefore, inevitably the roughening treatment or so-called black-oxide treatment to improve the bonding strength with the prepregs should be done by a mechanical process (such as grinding) or by using chemicals only without applying electricity.
The conventional processes are therefore insufficient and need longer processing time as compared with electrochemical processes which perform the roughening treatment efficiently with electricity applied to the circuit board.
Further, when a copper foil is subjected to a melanism treatment with copper sulfate, the bonding strength between the base of the copper foil and the treated layer is weak so that the treated layer is likely to peel off due to thermal expansion or contraction. The resultant printed circuit board has therefore a low reliability.